Catalytic reforming is a well established refinery process for improving the octane quality of naphthas or straight run gasolines. Reforming can be defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes, dehydroisomerization of alkylcyclopentanes, and dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst. In catalytic reforming, a multifunctional catalyst is usually employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, usually platinum, substantially atomically dispersed on the surface of a porous, inorganic oxide support, such as alumina. The support, which usually contains a halide, particularly chloride, provides the acid functionality needed for isomerization, cyclization, and hydrocracking reactions.
Reforming reactions are both endothermic and exothermic, the former being predominant, particularly in the early stages of reforming with the latter being predominant in the latter stages. In view thereof, it has become the practice to employ a reforming unit comprised of a plurality of serially connected reactors with provision for heating of the reaction stream from one reactor to another. There are three major types of reforming: semiregenerative, cyclic, and continuous. Fixed-bed reactors are usually employed in semiregenerative and cyclic reforming and moving-bed reactors in continuous reforming. In semiregenerative reforming, the entire reforming process unit is operated by gradually and progressively increasing the temperature to compensate for deactivation of the catalyst caused by coke deposition, until finally the entire unit is shut-down for regeneration and reactivation of the catalyst, In cyclic reforming, the reactors are individually isolated, or in effect swung out of line, by various piping arrangements. The catalyst is regenerated by removing coke deposits, and then reactived while the other reactors of the series remain on stream. The "swing reactors" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, which is then put back in the series. In continuous reforming, the reactors are moving-bed reactors, as opposed to fixed-bed reactors, with continuous addition and withdrawal of catalyst and catalyst is regenerated in a separate regeneration vessel.
Through the years, many process variations have been proposed to improve such things as C.sub.5.sup.+ liquid (a relatively high octane product stream) yield and/or octane quality of the product stream from catalytic reforming. For example, if a product of high octane is desired, e.g., 100 or higher RON (research octane number), the severity of reforming must be increased. This can generally be accomplished by reducing the space velocity or increasing reaction temperature. While increased severity for obtaining a higher octane product is desirable, it has disadvantages. For example, high severity usually: (i) reduces the yield of C.sub.5.sup.+ as a percent of the naphtha feedstock; (ii) usually causes more rapid accumulation of coke on the catalyst, thus rapidly decreasing the activity of the catalyst and requiring more frequent regeneration.
Practice of the present invention results in a significantly higher yield of hydrogen and of C.sub.5.sup.+ liquid as a percent of the naphtha feedstock. This is achieved by conducting the reforming in multiple stages and separating an aromatics-rich (high octane) stream between stages. The separation is performed after reforming at low severity, in a first stage or stages, to convert most of the alkycyclohexanes and alkylcyclopentanes to aromatics with minimum conversion, especially cracking, of paraffins. The remaining paraffin-rich, or aromatics-lean stream is processed in the downstream stage, or stages, at relatively high severity and preferably at relatively low pressures.
While there are some references in the art teaching aromatics removal between and after reactors of a reforming process unit, none suggests aromatics removal, after low severity catalytic reforming using a multimetallic catalyst followed by relatively high severity reforming, at low pressures.
For example, U.S. Pat. No. 2,970,106 teaches reforming to a relatively high octane (99.9 RON) followed by two stage distillation to produce three different streams: a light, intermediate, and heavy boiling stream. The intermediate stream, which contains C.sub.7 and C.sub.8 aromatics, is subjected to permeation by use of a semipermeable membrane resulting in an aromatics-rich stream and an aromatics-lean stream, both of which are distilled to achieve further isolation of aromatics. It is also taught that the aromatics-lean stream from the permeation process may be combined with a low octane stream from hydroformate distillation and further hydroformed, or isomerized, to improve octane number. It is further taught that the total hydroformate may be processed using the permeation process. Partial or low severity reforming, followed by aromatics separation, followed by further reforming with a stream containing a significant fraction of the paraffins in the original feedstock is not suggested in U.S. Pat. No. 2,970,106. Operation of the first-stage at high octane (99.9 RON) would result in very high conversion of feed paraffins. For example a key paraffin, n-heptane and its various isomers, would be about 46 to 54% converted at 99.9 RON for a petroleum naphtha cut (185.degree./330.degree. F.) comprised of 59% paraffins, 27% naphthenes, and 14% aromatics, which percents are liquid volume percent on total paraffins, naphthenes and aromatics present in the feed. In accordance with the process of the present invention, conversion of the n-heptane and its various isomers would be only about 11 to 14% in the first reforming stage-thus allowing more selective (less paraffin cracking) conversion to aromatics in the lower pressure second-stage.
Also, U.S. Pat. No. 3,883,418 teaches reforming a feedstock in the presence of hydrogen over a bifunctional catalyst in a first stage to convert naphthenes to aromatics, followed by distillation of the first stage product to produce an intermediate boiling (120.degree.-260.degree. F.) material which is subjected to extractive distillation to produce an aromatics-rich extract and an aromatics-lean raffinate. The aromatics-lean, or paraffins-rich, raffinate is then reformed in the presence of steam over a steam-stable catalyst. Steam reforming employs a steam reaction atmosphere in the presence of a catalyst having a relatively low surface area aluminate support material. Reforming, in accordance with the present invention, employs a hydrogen reaction atmosphere, in the substantial absence of steam, and in the presence of a catalyst having a relatively high surface area support material, such as gamma alumina or a zeolitic material.
Further, U.S. Pat. No. 4,206,035 teaches a process similar to U.S. Pat. No. 3,883,418, except that solvent extraction is used to remove aromatics instead of extractive distillation, and the aromatics-lean fraction sent to steam reforming is restricted to carbon numbers between 5 and 9. Also, specific hydrogen to hydrocarbon ratios and steam to hydrocarbon ratios are required.
U.S. Pat. No. 2,933,455 teaches a catalytic reforming process wherein the entire feedstock is first fractionated. The resulting 140.degree. to 210.degree. F. and 260.degree. to 420.degree. F. fractions are reformed in the presence of hydrogen in parallel reformers. In the reforming of the 140.degree. to 210.degree. F. fraction, the reforming severity is set such that naphthenes are converted to benzene and toluene and the resulting reformate is treated to remove aromatics. The remaining stream, containing at least 80 percent paraffins (primarily those containing 6 and 7 carbon atoms) is blended with the heavy 260.degree. to 420.degree. F. fraction and reformed in a second reformer. This reference teaches restricting the hydrocarbons reformed prior to aromatics removal to only the light naphtha components which form C.sub.6 and C.sub.7 aromatics. In addition, it teaches further reforming of the light paraffin-rich stream remaining after aromatics removal, in admixture with a heavy feed which is rich in aromatics and naphthenes.
Further, U.S. Pat. No. 3,640,818 teaches a process wherein virgin and cracked naphthas are reformed in a first stage and the reaction stream passed to solvent extraction where aromatics are removed. The paraffin-rich raffinate is passed to second stage reforming, preferably at pressures the same or higher than the first stage.